In recent years the mobile communication equipment, such as portable telephones and portable information terminal equipment, has come into widespread use. An oscillation circuit built in a semiconductor integrated circuit for the mobile communication is such an inverter oscillation circuit as disclosed in Japanese Patent Kokai Publication No. 2000-299612. FIG. 6 shows, by way of example, a conventional inverter oscillation circuit. In the illustrated circuit, an output terminal 11 of a signal inverting amplifier 10 with its input and output interconnected via a high resistance R11 is grounded via a capacitor C11 and is connected via a piezoelectric vibrator Xtal and a frequency adjusting capacitor C14 to an input terminal 12 of the signal inverting amplifier 10, and the input terminal 12 is grounded via a capacitor C12. The output signal from the circuit is provided via a series circuit of a resistor R12 and a capacitor C13. The piezoelectric oscillator usually employs a CMOS inverter that has enough driving ability to start piezoelectric oscillation in a short time. On this account, although the piezoelectric oscillation circuit does not inherently require such high driving power for its steady-state oscillation after being started, the operation of the CMOS inverter of high driving power inevitably causes unnecessary power consumption, giving rise to the problem of shortened battery life of the mobile communication equipment. Moreover, the use of the CMOS inverter excites the piezoelectric element strongly more than required during the stead-state oscillation and hence produces therein great mechanical stress—this may sometimes constitute an obstacle to achieving excellent aging characteristics of the piezoelectric oscillator.
Further, the conventional circuit has the piezoelectric vibrator Xtal and the frequency adjusting capacitor C14 inserted between the input and output of the signal inverting amplifier 10 for oscillation; the circuit oscillates in the vicinity of the resonance point of the piezoelectric vibrator Xtal by setting the gain of the signal inverting amplifier 10 at a large value and increasing the impedance between its input and output. That is, because of a small load capacitance across the piezoelectric vibrator Xtal, the conventional circuit may, in some cases, encounter the problem of incapability of adjusting frequency over a wide range in response to variations in the capacitance value of the capacitor C14.